1. Field of the Invention
The present invention relates to a method of equalization in a digital mobile communication such as an automobile telephone.
2. Description of the Background Art
In recent years, numerous researches have been carried out for the practical realization of digital mobile communication systems. In particular, for a digital automobile telephone system, a so called TDMA (Time Division Multiple Access) system is most likely to be adopted in Japan, U.S.A., and Europe.
Conventionally, such a TDMA system has been unable to achieve a good communication quality in a straight-forward manner because of a large influence of multi-path due to the high speed transmission speed, so that a use of equalizers has been indispensable.
However, in a conventional equalizer, a tracking of the variations of the transmission path characteristics by the equalizer becomes difficult when the Rf frequency to be utilized is high or a moving speed of the mobile station is high, such that it becomes difficult to achieve a good error rate characteristic.
Thus, there has been a problem that it has been difficult to achieve the sufficient tracking ability of the equalizer by using a conventional equalizer. This conventional problem persists even when the equalizer to be used is a decision feedback equalizer using an RLS (Recursive Least Square) algorithm which is known to have the best tracking ability among the currently available equalizers.
Furthermore, the conventional equalizer has problems that the convergence of the tap coefficients in the training is insufficient and that the error becomes large under a so called non-minimum phase condition because of a finite tap length.
In order to cope with these problems of a conventional equalizer, there has been a proposition of a bi-directional equalization scheme by A. Higashi and H. Suzuki of the NTT Radio Communication Systems Laboratories in Yokosuka, Japan. This bi-directional equalization scheme is applicable for a case in which training sequences for carrying out the initial training of the equalizer tap coefficients are attached in front and back of the data to be demodulated and equalized.
Namely, in this bi-directional equalization scheme, the equalization is carried out in the following steps. First, by using the training sequence attached in front of the data to be demodulated and equalized, the initial training of the equalizer tap coefficients is carried out, and then a forward equalization in which the received signals are inputted, demodulated and equalized in the equalizer in the same order (forward direction) as they were received is carried out as shown in FIG. 1A. Secondly, the received signals are temporarily stored in a memory, and by using the training sequence attached behind the data to be demodulated and equalized the initial training of the equalizer tap coefficients is carried out, and then a backward equalization in which the temporarily stored received signals are read out from the memory and inputted, demodulated and equalized in the equalizer in the opposite order (backward direction) as they were received is carried out as shown in FIG. 1B. Finally, the equalization output obtained by either one of the forward equalization and the backward equalization for which the equalization error according to the equalization error signals obtained in the equalizer during the forward and backward equalizations is smaller is selected as the final output.
However, as these authors noted themselves, in this bi-directional equalizations scheme, the sufficient error rate is not necessarily obtainable as there are cases in which the error rate becomes large even when the equalization error is small. Such a situation can be described as follows.
Namely, a typical decision feedback equalizer conventionally used has a configuration shown in FIG. 2, where the decision feedback equalizer comprises a feedback tap 116 with a feedback coefficient C5, a plurality of feed-forward taps 117 (117-1, 117-2, 117-3) with feed-forward coefficients C1 to C4, an adder 118, a subtractor 119 and a decision device 111. The feed-forward taps 117 provide a forward part of the equalizer while the feedback tap 116 provides a feedback part of the equalizer, which are added together by the adder 118 to reconstruct the transmitted signal without the multi-path distortion. The decision device 111 determines the binary values of the transmitted signal, i.e., which portion is 0 and which portion is 1 in the transmitted signal, according to the output of the adder 118. The output of the decision device 111 is fed back to the feedback tap 116. Meanwhile, the subtractor 119 subtracts the output of the adder 118 from the output of the decision device 111 to obtain the equalization error signal e(t), so as to assess the appropriateness of tap coefficients given to the feed-forward taps 117 and the feedback tap 116. The tap coefficient of each of the feed-forward taps 117 and the feedback tap 116 are adjusted according to this equalization error signal e(t). Further detail description of the decision feedback equalizer can be found in "Adaptive Equalization", S. U. H. Qureshi, Proceeding of the IEEE, Vol. 73, No. 9, pp. 1349-1987, September, 1985.
Now, in such a decision feedback equalizer, as the convergence characteristic of the equalizer deteriorates, the equalizer frequently tends to operate in a so called limit cycle state in which all the feed-forward tap coefficients C1 to C4 take the value equal to zero. In this limit cycle state, the equalizer appears as if it has a configuration shown in FIG. 3, where all the feed-forward taps 117 are disconnected such that the equalization error signal also takes a value equal to zero, which is totally unrelated to the input signals so that the reliability of the equalizer operation is totally lost.
Thus, in the above described bi-directional equalization scheme, the sufficient error rate is not necessarily obtainable because the selection of the final output may have to be made on a basis of such a totally unreliable equalization error signal e(t).
This problem related to the error rate in the bi-directional equalization scheme can also be seen from the experimental result reported by these authors, which is shown in FIG. 4. As can be seen in FIG. 4, the bi-directional equalization scheme is unable to achieve any significantly lower bit error rate compared with the cases using only either one of the forward equalization and backward equalization.